1. Field of the Invention
The present invention relates to a code division multiple access communication system for assigning different spread spectrum codes that have high self correlation coefficients and much higher transmission rates than transmission data to individual transmission channels, directly multiplying the transmission data by spread spectrum data as the spread spectrum codes, directly spreading the transmission data to a wide band on the frequency axis, and multiplexing a plurality of channels to the same frequency band, in particular, to a gain compensating apparatus for use with an amplifying portion for compensating the gain of a power amplifying portion in a radio portion corresponding to transmission power information designated for stabilizing for example a cell radius of a mobile communication.
2. Description of the Related Art
As factors with respect to deterioration of the gain of a transmission amplifier, there are a temperature fluctuation and an aged tolerance. As countermeasures against the temperature fluctuation, for example, the following two systems are known. As a first system, a device whose characteristic varies depending on the temperature such as a thermistor is disposed in an objective circuit. As a second system, a transmission output signal is detected. The detected data is compared with a desired output signal and the gain of an amplifier is controlled with a variable gain amplifier or a variable attenuator disposed on a signal line so that the voltage of the detected data matches the voltage of the desired output signal.
In the second system, the deterioration of the gain due to the aged tolerance can be also compensated. A technology for compensating the gain of a transmitting portion has been disclosed as Japanese Patent Laid-Open Publication No. 8-139617, titled "Transmission Output Signal Controlling Circuit for use with AME Transmitter (translated title)", by Yasutaka et al. In this related art reference, the fluctuation of the gain of the amplifier in the AME transmitter is suppressed against fluctuations of frequency and temperature thereof so as to always accomplish the maximum modulation factor of 100%. As means for accomplishing such an object, the level of a local oscillation signal generated for converting a modulation signal into a transmission frequency signal is compared with the level of the transmission output signal and the difference is fed back so as to adjust the gain of the amplifier. In the related art reference, a stable oscillation signal is required in any condition and in any environment.
Thus, as with the code division multiple access communication system, when the level of the modulation signal that is input to the amplifying portion fluctuates depending on the number of multiplexed transmission channels, there is no means for varying a gain control condition. In addition, to distinguish the cause of the fluctuation of the gain from the fluctuation of the transmission output signal corresponding to the fluctuation of the number of multiplexed transmission channels, the fluctuation of the power in the case that the power of each transmission channel is independently controlled, the environmental fluctuation of the transmission amplifier, and the aged tolerance thereof, information of a power to be currently transmitted should be obtained. However, to obtain information necessary for compensating the gain, data should be exchanged with so-called host units such as a central monitoring/controlling portion and a signal generating block of each transmission channel.
The above-described related art reference has the following four problems. As the first problem, since a highly stable local oscillation signal source is used for a reference signal, the level of the local oscillation signal, the level of the signal that is input to a comparator, the level of the output signal of the comparator, and so forth cannot be varied against the information of the transmission power. Thus, the gain of the amplifier according to the code division multiple access communication system cannot be compensated.
As the second problem, there are a plurality of transmission channels per RF carrier at a time. In addition, powers of individual transmission channels are independently controlled. Thus, to allow the amplifying portion to control the gain thereof, it should obtain the power to be currently transmitted. As means for obtaining the transmission power, information such as the number of transmission channels and power control state of each transmission channel may be obtained from such host units. In this system, when power control intervals are short, the gain control does not follow the power control.
As the third problem, in the case that to solve the second problem, a storing device may be disposed in the gain compensating circuit and thereby information received from the host units are stored for a predetermined time period so as to decrease the delay of the control. However, as with the second problem, since the process speed is restricted, when the power control period intervals are short, the gain control does not follow the power control.
As the fourth problem, since the host units should inform the radio portion of a power to be transmitted per RF carrier to the radio portion, a dedicated gain compensating circuit is required. Thus, the scale of the apparatus becomes large. In addition, the dedicated gain compensating circuit and many circuits should be connected and thereby the number of connections becomes large.